Update:
Installed Safari 15.1 Beta 4.
Added YouTube bookmark = works.
Added multiple YouTube bookmarks = works.
cmd + d YouTube bookmark = works.
Spec: MacBook Air M1 (2020).
OS: macOS Big Sur 11.6
Browser: Safari (Version 15.1 (16612.2.9.1.22, 16612))
Never thought I'd be so happy so get such basic functionality as adding a YouTube bookmark back working.
Good job, Apple

P.S. Link -
https://developer.apple.com/safari/download/ (please note you'll need an Apple ID to download).
Initial post:
Safari 15.1 Beta 2 crashes every single time.
Haven't tried beta 3 or beta 4 yet.
Not sure what is more suprising:
1) Apple released Safari 15 without first fixing this (surely Apple employees visit YouTube).
or
2) It's been a month since the release of the Safari 15 and who knows when Apple will fix this bug.
And the only way to revert to Safari 14 is to reinstall macOS.